1. Field of the Invention
The present invention relates to an optical information recording medium capable of optically recording and reading information. More particularly, it relates to an optical information recording medium capable of optically recording and reading electronic information and further, capable of favorably recording visual information on a label face side by means of a laser beam.
2. Discussion of Background
Of an electronic information recording medium for recording, storing and rewriting a lot of information, importance as computer peripheral equipment is increasingly high in recent years. Particularly, an optical information recording medium on which recording and reading are carried out by means of a laser beam attracts attention as a mass storage recording medium since high density recording, storing and reading are possible. As examples of such an optical information recording medium, a phase-change type optical recording medium represented by CD-RW or DVD-RW and an organic dye type optical recording medium represented by CD-R or DVD-R may, for example, be mentioned.
The content of electronic information recorded in such an optical information recording medium can be confirmed by optical reading. On the other hand, it is important to display the content of electronic information as visual information on the surface of the medium in view of information management. Heretofore, as a display method therefor, a method of recording by printing the content of electronic information on a label face (the face opposite to the face on which electronic information is to be recorded) of a medium by means of an ink jet printer or a thermal transfer printer, or a method of bonding e.g. a label on which the content of electronic information is recorded to a label face of a medium, may, for example, be employed. Further, in recent years, a method of recording visual information on a visual information recording layer formed on a label face side of a medium by irradiation with a laser beam has been reported (e.g. JP-A-2000-173096, JP-A-2001-283464), which is expected as a method of recording visual information without exerting mechanical influences over a medium.
The above method of recording visual information on a label face side of a medium by means of a laser beam, as reported in JP-A-2000-173096 and JP-A-2001-283464, has the following drawbacks. Namely, the method of recording visual information on a label face side of a medium by means of a laser beam utilizes a change such as a reaction, melting or precipitation due to heat, and thus excessive irradiation with a laser beam will have a thermal influence or the like over the electronic information recorded on a medium, and accordingly careful attention has to be paid to an irradiation method. Accordingly, in order to reduce a thermal influence over a medium, it is necessary to use a laser beam with a low output at a level of from 10 to 200 mW. When such a low output laser beam is used, it is effective to efficiently concentrate the energy of the laser beam by condensing the low output laser beam on a layer (visual information recording layer) on which visual information is to be recorded, and for that purpose, it is considered necessary to apply a focus servo.
Further, in order to efficiently concentrate the low output laser beam on the visual information recording layer and to stably apply a focus servo, a reflective layer to obtain a reflected light to be utilized for focusing and a layer (laser beam transparent layer) comprising a material transparent to a laser beam, which protects the visual information recording layer from physical scratching or the like and which efficiently focuses the laser beam on the visual information recording layer may be formed in some cases.
However, in such a case, if the surface of the visual information recording layer or the laser beam transparent layer has irregularities, such a phenomenon occurs that the laser beam applied from the label face side is diffused on the surface of the visual information recording layer or the laser beam transparent layer. If the laser beam is diffused in such a manner, the amount of light which enters the visual information recording layer decreases, and the visual information can not favorably be recorded. Further, stable focusing tends to be difficult due to the diffusion.
The present invention has been made to solve technical problems which showed up when visual information is recorded on a visual information recording layer formed on a label face side of an optical information recording medium by means of a low output laser beam. Accordingly, the object of the present invention is to provide an optical information recording medium on which visual information can be recorded on a label face side by using a laser apparatus for recording and reading electronic information by irradiation with a low output laser beam without exerting bad influences over electronic information recorded on the medium.
In order to achieve such an object, the optical information recording medium of the present invention employs such a structure that an outermost layer such as a laser beam transparent layer having a reduced surface roughness is formed as the outermost layer on the label face side. Namely, the optical information recording medium of the present invention comprises a substrate and a visual information recording layer which is formed directly or via another layer on the substrate and on which visual information is to be recorded by means of a laser beam to be applied from a label face side on the opposite side from the substrate side, and is characterized in that the surface roughness of an outermost layer on the label face side is at most ½ of the wavelength of the laser beam to be applied from the label face side. Further, the surface roughness of the outermost layer is preferably at most ¼ of the wavelength of the laser beam.
Further, the optical information recording medium of the present invention is an optical information recording medium comprising a substrate having guide grooves and/or pits for recording and reading, and at least an electronic information recording layer on which digital information is to be recorded by means of a laser beam to be applied from the substrate side, a reflective layer and a visual information recording layer on which visual information is to be recorded by means of a laser beam to be applied from a label face side on the opposite side from the substrate side, laminated in this order on the substrate, characterized in that the surface roughness of an outermost layer on the label face side is at most ½ of the wavelength of the laser beam to be applied from the label face side. Further, the surface roughness of the outermost layer is preferably at most ¼ of the wavelength of the laser beam.
Further, the optical information recording medium of the present invention comprises a substrate, a visual information recording layer which is formed on the substrate and on which visual information is to be recorded by means of a beam to be applied from a label face side on the opposite side from the substrate side, and an overcoat layer formed on the label face side of the visual information recording layer, and is characterized in that the surface roughness of the overcoat layer is at most ½ of the wavelength of the beam.
The optical information recording medium of the present invention preferably has an overcoat layer as an outermost layer. Further, the overcoat layer is formed preferably by a material transparent to a laser beam, and the overcoat layer is formed preferably so that the lower limit of the thickness of the overcoat layer is 10 μm, and the upper limit is 100 μm. Further, the overcoat layer is formed preferably to have a refractive index of at least 0.4 and at most 2.6. Further, the visual information recording layer is characterized by being formed by such a material that its color developing properties or its transparency changes by a laser beam applied to a visual information recording layer.
Further, the optical information recording medium of the present invention is an optical information recording medium comprising a substrate, and an electronic information recording layer on which digital information is to be recorded by means of a laser beam to be applied from the substrate side, a reflective layer formed on the opposite side of the electronic information recording layer from the substrate side, and a protective layer formed on the opposite side of the reflective layer from the substrate side, formed on the substrate, characterized by having a visual information recording layer which is formed on the opposite side of the protective layer from the substrate side and on which visual information is to be recorded by means of a laser beam to be applied from a label face side on the opposite side from the substrate side, and an overcoat layer formed on the label face side of the visual information recording layer and having a surface roughness of at most ½ of the wavelength of the laser beam to be applied from the label face side.
Still further, the present invention provides a process for producing an optical information recording medium, which comprises a step of forming on a substrate an electronic information recording layer on which digital information is to be recorded by means of a beam to be applied from the substrate side, a step of forming a reflective layer as an upper layer of the formed electronic information recording layer, a step of forming a protective layer as an upper layer of the formed reflective layer, a step of forming as an upper layer of the formed protective layer a visual information recording layer on which visual information is to be recorded by means of a laser beam to be applied from a label face side on the opposite side from the substrate side, and a step of forming as an upper layer of the formed visual information recording layer an overcoat layer having a surface roughness of at most ½ of the wavelength of a laser beam to be applied from the label face side by a wet film formation method.
In the present invention, “visual information” represents information which is visually read, such as characters, symbols, images such as illustrations and photographs, and geometric patterns, and “electronic information” represents information, the content of which is read by some reading device, such as information recorded by digital signals.
According to the present invention, an optical information recording medium on which visual information can be recorded on a label face by means of irradiation with a low output laser beam at a level of from 10 to 200 mW can be provided.